Most of the user equipment (UE) devices, e.g., smartphones, operate in connected mode for most of the operating time due to frequent use of data.
FIG. 1 is a diagram showing an example of a case where INVITE messages are transmitted from a number of conventional UE devices in connected mode.
Referring to FIG. 1, when a number of UE devices 110 are in connected mode, they may simultaneously transmit an INVITE message to initiate a Voice over Long Term Evolution (VoLTE) call, although the message is small in terms of data amount. That is, when a number of UE devices 110 all transmit an INVITE message, a number of invite message are gathered simultaneously due to a Proxy-Call Session Control Function (P-CSCF) and Internet Protocol Multimedia Service (IMS) 120 may be inoperative. In particular, when it is assumed that all users or UE devices 110, registered on P-CSCF, initiate a call simultaneously, increasing the capacity of P-CSCF causes cost issues. To resolve this, the capacity of P-CSCF and IMS entities is supported, so that calls corresponding to a certain percentage of users 110 that have registered their calls simultaneously can be simultaneously made. In this state, a particular situation where invite messages are simultaneously created from a number of UE devices as shown in FIG. 1 may occur, e.g., increase in calls due to a natural disaster, increase in calls due to a particular sports event, etc. This may result in an increase load upon IMS entities 120 such as P-CSCF, etc.; therefore, rendering the IMS inoperative and also causing situations in which an emergency call cannot be processed or a high priority call cannot be processed by a person on duty at a police station or a fire station.